It is known to use Time to Digital Converters (“TDC”) and Analogue to Digital Converters (“ADC”) as part of data recording electronics for many analytical instruments including Time of Flight (“TOF”) mass spectrometers.
Time of Flight instruments incorporating Time to Digital Converters are known wherein signals resulting from ions arriving at an ion detector are recorded. Signals which satisfy defined detection criteria are recorded as a single binary value and are associated with a particular arrival time relative to a trigger event. A fixed amplitude threshold may be used to trigger recording of an ion arrival event. Ion arrival events which are subsequently recorded resulting from subsequent trigger events are combined to form a histogram of ion arrival events. The histogram of ion arrival events is then presented as a spectrum for further processing. One disadvantage of TDCs is that once an ion arrival event has been recorded then there is a significant time interval or dead-time following the ion arrival event during which time no further ion arrival events can be recorded. Another important disadvantage of TDCs is that they are unable to distinguish between a signal resulting from the arrival of a single ion at the ion detector and a signal resulting from the simultaneous arrival of multiple ions at the ion detector. This is due to the fact that the signal will only cross the threshold once, irrespective of whether a single ion arrived at the ion detector or whether multiple ions arrived simultaneously at the ion detector. Both situations will result in only a single ion arrival event being recorded. These effects have the result of limiting the dynamic range of the ion detector system.
In Time of Flight instruments which incorporate ADCs, an ADC is arranged to digitise signals resulting from ions arriving at an ion detector relative to a trigger event. The digitised signals resulting from subsequent trigger events are summed or averaged to produce a spectrum for further processing. One advantage of using an ADC as part of an ion detector system is that multiple ions which arrive substantially simultaneously at an ion detector and at relatively higher signal intensities can be recorded without the ion detector suffering from distortion or saturation effects.
Typically, an ADC based system measures the time of flight transient “start” events i.e. the time at which ions are accelerated into the flight tube of the Time of Flight analyser. A detector, ADC and associated processing hardware are capable of reducing the detector signal for each time of flight transient to “stop” events i.e. a series of time and intensity pairs representing the arrival of a single ion or the simultaneous arrival of a plurality of ions at the detector subsequent to a start event. This series is naturally ordered in time.
The time and intensity pairs may be organised in a histogram with results being put into a series of windows along a time axis. The histogramming algorithm typically used needs to perform Read Modify Write (“RMW”) cycles on memory in which data is stored, each time that a new event arrives. Synchronous dynamic random access memory (“SDRAM”) often requires tens of clock cycles to achieve a single access. These clock cycles are further extended when doing a bus turnaround i.e. switching between a read and a write. The result is that using a conventional histogramming algorithm limits the achievable performance for SDRAM.
US 2011/0024620 (Hidalgo) describes a method of accumulating ion intensities in Time of Flight (TOF) bins. In the method described in Hidalgo, data structures that each relate to an ion detection event are generated, wherein the ion detection event can relate to plural temporally-irregular and interfering transients. The method described in Hidalgo is intended to resolve the data for these interfering transients.
It is desired to provide an improved method of data acquisition for a Time of Flight mass spectrometer.